113 lines
3.3 KiB
Groff
113 lines
3.3 KiB
Groff
.\" $NetBSD: hz.9,v 1.9 2010/03/26 19:40:41 wiz Exp $
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.\"
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.\" Copyright (c) 2001 The NetBSD Foundation, Inc.
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.\" All rights reserved.
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.\"
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.\" This code is derived from software contributed to The NetBSD Foundation
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.\" by Thomas Klausner.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
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.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
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.\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
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.\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
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.\" BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
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.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
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.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
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.\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
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.\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
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.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
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.\" POSSIBILITY OF SUCH DAMAGE.
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.\"
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.Dd March 25, 2010
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.Dt HZ 9
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.Os
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.Sh NAME
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.Nm hz ,
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.Nm tick ,
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.Nm tickadj ,
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.Nm stathz ,
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.Nm profhz
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.Nd system time model
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.Sh SYNOPSIS
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.In sys/kernel.h
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.Pp
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.Vt extern int hz;
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.Vt extern int tick;
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.Vt extern int tickadj;
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.Vt extern int stathz;
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.Vt extern int profhz;
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.\" XXX: .Vt extern int schedhz; ?
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.Sh DESCRIPTION
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The essential clock handling routines in
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.Nx
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are written to operate with two timers that run independently of each other.
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The main clock, running
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.Va hz
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times per second, is used to keep track of real time.
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.Pp
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In another words,
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.Va hz
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specifies the number of times the
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.Xr hardclock 9
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timer ticks per second.
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Normally
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.Xr hardclock 9
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increments time by
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.Va tick
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each time it is called.
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If the system clock has drifted,
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.Xr adjtime 2
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may be used to skew this increment based on the rate of
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.Va tickadj .
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.Pp
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The second timer is used to gather timing statistics.
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It also handles kernel and user profiling.
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If the second timer is programmable,
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it is randomized to avoid aliasing between the two clocks.
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The mean frequency of the second timer is
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.Va stathz .
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If a separate clock is not available,
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.Va stathz
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is set to
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.Va hz .
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.Pp
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If profiling is enabled, the clock normally used to drive
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.Va stathz
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may be run at a higher rate
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.Va profhz ,
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which is required to be a multiple of
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.Va stathz .
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This will give higher resolution profiling information.
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.Pp
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These system variables are also available as
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.Em struct clockinfo
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from
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.Xr sysctl 3
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and
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.Sy kern.clockrate
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from
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.Xr sysctl 8 .
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The
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.Va hz
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is hardware-dependent; it can be overridden
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(if the machine dependent code supports this) by defining
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.Dv HZ
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in the kernel configuration file (see
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.Xr options 4 ) .
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Only override the default value if you really know what you are doing.
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.Sh SEE ALSO
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.Xr adjtime 2 ,
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.Xr callout 9 ,
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.Xr hardclock 9 ,
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.Xr microtime 9 ,
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.Xr time_second 9
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